Electrochemical energy conversion devices and cells, and negative electrode-side materials for them

ABSTRACT

An electrochemical energy conversion device  10  comprising a stack of solid oxide electrochemical cells  12  alternating with gas separators  14, 16 , wherein scavenger material selected from one or both of free alkali metal oxygen-containing compounds and free alkaline earth metal oxygen-containing compounds is provided in or on one or more of the negative electrode-side of the cell  12 , the adjacent gas separator  16  and any other structure of the device  10  forming a gas chamber  66  between the cell and the gas separator. The invention also extends to the treated cell  12.

PRIORITY

This application claims priority from Australian Provisional PatentApplications 2014900069 and 2014900070 each filed on 9 Jan. 2014 (thepriority applications), and the entire content and disclosure of each ofthose provisional patent applications is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to electrochemical energy conversiondevices as well as to solid oxide electrochemical cells for them, and isparticularly concerned with reducing degradation in the electrochemicalperformance of the cell s and devices.

BACKGROUND ART

Electrochemical energy conversion devices include fuel cell systems aswell as hydrogen generators and other electrolysers, such as forco-electrolysing water and CO₂.

Fuel cells convert gaseous fuels (such as hydrogen, natural gas andgasified coal) via an electrochemical process directly into electricity.A fuel cell continuously produces power when supplied with fuel andoxidant, normally air. A typical fuel cell consists of an electrolyte(ionic conductor, H⁺, O²⁻, CO₃ ²⁻ etc.) in contact with two electrodes(mainly electronic conductors). On shorting the cell through an externalload, fuel oxidises at the negative electrode resulting in the releaseof electrons which flow through the external load and reduce oxygen atthe positive electrode. The charge flow in the external circuit isbalanced by ionic current flows within the electrolyte. Thus, at thepositive electrode oxygen from the air or other oxidant is dissociatedand converted to oxygen ions which migrate through the electrolytematerial and react with the fuel at the negative electrode/electrolyteinterface. The voltage from a single cell under load conditions is inthe vicinity of 0.6 to 1.0 V DC, and current densities in the range of100 to 1000 mAcm⁻² can be achieved. In addition to the electricity,water is a product of the fuel cell reaction. Hydrogen generators andother electrolysers may be considered as fuel cell systems operating inreverse. Thus, a hydrogen generator produces hydrogen and oxygen whenelectricity and water are applied to the electrochemical cell.

A fuel cell system capable of producing electricity may be designed torun in reverse in order to produce hydrogen, for example producingelectricity during the day and hydrogen at night, with the hydrogenoptionally being stored for use the next day to produce moreelectricity. However, it may be advantageous from the efficiencyperspective to design separate fuel cell systems and hydrogengenerators. While the invention is concerned with electrochemical energyconversion devices generally, for convenience only it will be describedhereinafter primarily with reference to electricity generating fuel cellsystems and cells for them.

Several different types of fuel cells have been proposed. Amongst these,solid oxide fuel cell systems (SOFC) are regarded as the most efficientand versatile power generation system, in particular for dispersed powergeneration, with low pollution, high efficiency, high power density andfuel flexibility, and the invention is particularly concerned with solidoxide electrochemical energy conversion cells and with devices usingthem. Numerous SOFC configurations are under development, includingtubular, monolithic and planar designs, and are now in production. Theplanar or flat plate design is perhaps the most widely investigated andnow in commercial use, and the invention is particularly concerned inone aspect with electrochemical energy conversion devices comprising astack of such solid oxide electrochemical cells. However, in anotheraspect, the invention also extends to solid oxide electrochemical energyconversion cells generally, that is it is concerned with tubular cellsand monolithic cells, as well as with planar cells.

For convenience only, the invention will be further described solelywith respect to planar or flat plate design solid oxide electrochemicalenergy conversion cells, and devices using them. In these devices,individual planar SOFCs comprising electrolyte/electrode laminatesalternate with gas separators, called interconnects when the gasseparators convey electricity from one SOFC to the next, to formmulti-cell units or stacks. Gas flow paths are provided between the gasseparators and respective electrodes of the SOFCs, for example byproviding gas flow channels in the gas separators, and the gasseparators maintain separation between the gases on each side. Apartfrom having good mechanical and thermal properties, as well as goodelectrical properties in the case of interconnects and goodelectrochemical properties in the case of the fuel cells themselves, theindividual fuel cell device components must be stable in demanding fuelcell operating environments. SOFCs operate in the vicinity of 600°C.-1000° C. and, for devices using them to be economical, typicallifetimes of 5-6 years or more of continuous operation are desired.Thus, long term stability of the various device components is essential.Only a few materials fulfil all the requirements. In general, the highoperating temperature of the SOFCs, the multi-component nature of thedevices and the required life expectancy of several years severelyrestricts the choice of materials for the fuel cells, gas separators andother components such as seals, spacer plates and the like.

A variety of different materials have been proposed for SOFC gasseparators, including ceramic, cermet and alloys. For electricallyconductive gas separators, that is interconnects, metallic materialshave the advantage generally of high electrical and thermalconductivities and of being easier to fabricate. However, stability in afuel cell environment, that is high temperatures in both reducing andoxidising atmospheres, limits the number of available metals that can beused in interconnects. Most high temperature oxidation resistant alloyshave some kind of built-in protection mechanism, usually formingoxidation resistant surface layers. Metallic materials commonly proposedfor high temperature applications include, usually as alloys, Cr, Al andSi, all of which form protective layers. For the material to be usefulas an interconnect in SOFC devices, any protective layer which may beformed by the material in use must be at least a reasonable electronicconductor. However, oxides of Al and Si are poor conductors. Therefore,alloys which appear most suitable for use as metallic interconnects inSOFCs, whether in cermet or alloy form, contain Cr in varyingquantities.

Cr containing alloys form a layer of Cr₂O₃ at the external surface whichprovides oxidation resistance to the alloy. The formation of a Cr₂O₃layer for most electrical applications is not a problem as it hasacceptable electrical conductivity. However, for SOFC applications, amajor problem is the high vapour pressure and therefore evaporation ofoxides and oxyhydroxides of Cr Cr⁶⁺) on the positive electrode side ofthe fuel cell at the high operating temperatures. At high temperatures,oxides and oxyhydroxides of Cr (Cr⁶⁺) are stable only in the gas phaseand have been found to react with positive electrode materials leadingto the formation of new phases such as chromates, which destroy theelectrode material and make it electrically resistive, as well as todeposits of Cr₂O₃ on the electrolyte. These reactions very quicklyreduce electrode activity to the oxygen reduction reaction at andadjacent the positive electrode/electrolyte interface, and therebyconsiderably degrade the electrochemical performance of the cell.

It has been attempted to alleviate this problem of degradedelectrochemical performance by coating the positive electrode side ofthe interconnect with a perovskite barrier layer such as strontium-dopedlanthanum manganite (LaMnO₃) (LSM), which may also be the material ofthe positive electrode, but while short term performance was maintainedthere continued to be an unacceptable long term degradation inperformance.

The problem of degradation due to evaporation of oxides andoxyhydroxides of Cr from chromium-containing materials on the positiveelectrode side of the fuel cell was greatly relieved by the inventiondescribed in the applicant's WO96/28855, that is forming aself-repairing coating on the positive electrode side of achromium-containing interconnect, the coating comprising an oxidesurface layer comprising at least one metal M selected from the groupMn, Fe, Co and Ni and a M, Cr spinel layer intermediate thechromium-containing substrate of the interconnect and the oxide surfacelayer. Such a coating may also be formed on other chromium-containingheat resistant steel surfaces that are on the positive electrode side ofthe plant. However, it remains a challenge to ensure the coating remainsfully dense to prevent the release of the chromium species in thedemanding fuel cell operating conditions.

Other solutions have also been proposed for alleviating the degradationin fuel cell performance due to evaporation of oxides and oxyhydroxidesof Cr on the positive electrode side of the fuel cell. For example, alow (or no) chromium steel is proposed in the applicant's WO00/75389, inwhich an alumina coating is formed on oxidation of the surface ratherthan chromium oxide. However, due to the low electrical conductivity ofalumina, this heat resistant steel composition is not suitable for gasseparators that are intended to act as interconnects conductingelectricity from one side to the other.

In a further effort to limit the problem of degradation due toevaporation of oxides and oxyhydroxides of Cr on the positive electrodeside of the fuel cell, it has been proposed to introduce another layer(referred to hereinafter as “shield layer”) on the positive electrodelayer to absorb chromium before it reaches the positive electrode layer.

Positive electrode materials for SOFCs are generally perovskites oroxides having perovskite-type structures (refined to herein as“perovskites”), such as lanthanum strontium manganite or LSM(La_(1-x)Sr_(x)MnO_(3-δ)), lanthanum strontium cobaltite or LSCo(La_(1-x)Sr_(x)CoO_(3-δ)), lanthanum strontium ferrite or LSF(La_(1-x)Sr_(x)FeO_(3-δ)), La_(1-x)Sr_(x)CO_(1-y)Fe_(y)O_(3-δ) (LSCF),LaNi_(x)Fe_(1-x)O_(3-δ) (LNF), and Ba_(1-x)Sr_(x)Co_(1-y)Fe_(y)O_(3-δ)(BSCF) where 0≦δ<1 depending on the dopant. Other examples includeSm_(x)Sr_(1-x)CoO_(3-δ) (SSC), La_(x)Sr_(1-x)Mn_(y)Co_(1-y)O_(3-δ)(LSMC), Pr_(x)Sr_(1-x)FeO_(3-δ) (PSF),Sr_(x)Ce_(1-x)Fe_(y)Ni_(1-y)O_(3-δ) (SCFN),Sr_(x)Ce_(1-x)Fe_(y)Co_(1-y)O_(3-δ), Pr_(x)Ce_(1-x)CO_(y)Fe_(1-y)O_(3-δ)and Pr_(x)Ce_(1-x)Co_(y)Mn_(1-y)O_(3-δ). In the strontium-containingperovskites, for example, the strontium is provided as a doping agentthat is bound into the perovskite structure.

The aforementioned shield materials proposed to date have beenperovskites, for example having a similar composition to the positiveelectrode layer but more reactive with chromium than the positiveelectrode material in order to absorb it before it reaches and reactswith the positive electrode layer. In one example where the positiveelectrode material is LSM the shield layer material is LSCo(La_(1-x)Sr_(x)CoO_(3-δ)), but other materials are possible.

Some barrier materials are proposed in the paper by Thomas Franco et al“Diffusion and Protecting Barrier Layers in a Substrate Supported SOFCConcept”, E-Proceedings of the 7th European Fuel Cell Forum, Lucerne(2006), P0802-051. This paper also sets out additional details on thereactions occurring.

Even with these advancements, it is found that degradation of fuel cellperformance remains a problem. This has led to extensive furtherinvestigations by the applicant as to the causes, from which additionalpositive electrode material poisons have been identified.

As a result of these investigations the applicant has found that sulphurpoisons the positive electrode of an SOFC in much the same way aschromium, by forming sulphate crystals with components of the electrodematerial, such as strontium, a d possibly destroying the chemicalstructure of the electrode material. It has been found that the sulphurmay be derived from the oxidant supply (generally air), usually in theform of SO₂, or from elsewhere in the system, for example in the glassseals used to seal the SOFCs and gas separators together or elsewhereupstream of the positive electrode-side chamber, where the sulphur maybe present as an impurity and appear as SO₂ or SO₃.

The further investigations have also shown that boron can act in thesame way as chromium and sulphur to poison the positive electrodematerial in the conditions of use. Boron may be present in the system asa compound of the glass seals, but may also be present in othercomponents of the fuel cell system exposed to the oxidant.

It is believed that other element present in the system components, orin the oxidant supply, whether as impurities or otherwise, may also bereacting with components of the positive electrode material andpoisoning the material. Possible examples of these elements includesilicon.

Alleviating reactions with the positive electrode material by poisons inthe system in use of an electrochemical energy conversion cell, andtherefore alleviating cell performance degradation, is an aim of theinvention described and claimed in a co-pending PCT patent applicationfiled by the applicant concurrently herewith and claiming priority fromthe priority applications, entitled “Electrochemical Energy ConversionDevices and Cells, and Positive Electrode-Side Materials for them”, thecontents of which are incorporated herein by reference.

However, the applicant's further investigations into the causes of fuelcell performance degradation has revealed that in addition to poisoningof the positive electrode material, the negative electrode side alsosuffers from performance degradation.

SOFC negative electrode materials are generally nickel based, mostcommonly Ni/YSZ cermets. Other nickel cermets being used as negativeelectrode materials include Ni/GDC (Ni/gadolinium doped ceria), Ni/SDC(Ni/samarium doped ceria), Ni/ScSZ (Ni/scandiastabilised zirconia) andNi/ScCeSZ (Ni/scandia ceria stabilised zirconia). Pt, Rh and Ru have allbeen used in place of nickel in cermet negative electrode materials, butthese metals are considerably more expensive than nickel and thereforemuch less common.

It is well known that sulphur reacts with nickel in negative electrodematerials under SOFC operating conditions to degrade the performance ofthe electrode, and for this reason sulphur is commonly removed from SOFCfuel sources. However, the applicant's further investigations have ledto a belief that, even if sulphur is removed from the fuel source,sulphur continues to degrade the negative electrode material. This isbelieved to be as a result of residual sulphur in the fuel or as aresult of sulphur from elsewhere in the system, for example in the glassseals used to seal the SOFCs and gas separator or elsewhere upstream ofthe negative electrode, where the sulphur may be present as an impurity.Some of the reasons for degradation of the negative electrode materialperformance due to sulphur are believed to be: at very low sulphurlevels, for example as low as 1 ppm in the gas stream, the electrodematerial can degrade due to surface adsorption of the sulphur on thenickel; at higher levels of sulphur, Ni—S alloys are formed; and at evenhigher levels of sulphur, nickel sulphides form.

The effect on SOFC anodic performance of hydrogen and hydrocarbon fuelscontaminated with up to 50 ppm wet H2S was investigated by Limin Liu etal, in the paper “Sulfur Tolerance Improvement of Ni-YSZ Anode byAlkaline Earth Metal Oxide BaO for Solid Oxide Fuel Cells”,Electrochemistry Communications 19 (2012) 63-66. In the paper it isreported that BaO infiltrated throughout the functional anode layer at alevel of about 5 wt % was found to enhance the sulphur tolerance abilityof the Ni—YSZ anode over the test period of 27 hours. It was concludedthat water played a very crucial role in this, and that this may resultfrom the good water dissociative absorption ability of BaO.

The applicant's further investigations on the negative electrode sidehave also identified that boron and phosphorus species from seals andother components of the device may be entering the atmosphere in thenegative electrode-side chamber and leading to performance degradationin some way. In the case of boron at least this appears to be bypromoting grain growth in the nickel or other metal of the electrodematerial. The phosphorus species may be reacting with the nickel andpoisoning it.

Other species that have been found to be detrimental to the negativeelectrode-side performance, possibly as a result of reacting with andthereby poisoning the nickel, are chlorine, siloxane and selenium. Thesemay be present on the negative electrode side as impurities, forexample, in the fuel gas or the glass used for the seals.

Another problem identified on the negative electrode side is theunintended ongoing sintering of nickel in porous layers in the negativeelectrode-side chamber, particularly but not only in the negativeelectrode-side structure of the electrochemical cell, including thenegative electrode material. This sintering leads to a loss of surfacearea in the porous layer or layers and a decrease of the triple phaseboundary area of the electrode layer, resulting in degradation inelectrochemical performance.

It is clear that it would be highly desirable to alleviate long-termdegradation of cell performance on the negative electrode side in use ofan electrochemical energy conversion cell.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan electrochemical energy conversion device comprising a stack of solidoxide electrochemical cells alternating with gas separators, whereineach electrochemical cell comprises a layer of solid oxide electrolyte,a negative electrode-side structure on one side of the electrolyte layerand comprising one or more porous layers including a functional layer ofnegative electrode material having an interface with the one side of theelectrolyte layer, and a positive electrode-side structure on theopposite side of the electrolyte layer and comprising one or more porouslayers including a layer of positive electrode material having aninterface with the opposite side of the electrolyte layer, wherein saidelectrochemical cell and a first of the gas separators on the negativeelectrode side of the electrochemical cell at least partly formtherebetween a negative electrode-side chamber and said electrochemicalcell and a second of the gas separators on the positive electrode sideof the electrochemical cell at least partly form therebetween a positiveelectrode-side chamber, and wherein chemically unbound material selectedfrom one or both of free alkali metal oxygen-containing compounds andfree alkaline earth metal oxygen-containing compounds is provided in oron one or more of the negative electrode-side structure, the first gasseparator and any other structure of the electrochemical energyconversion device forming the negative electrode-side chamber, theunbound material acting to reduce degradation of electrochemicalperformance on the negative electrode side of the electrochemical energyconversion device during use of the device, and wherein if thechemically unbound material is provided in the functional layer ofnegative electrode material there is no chemically unbound materialpresent at the interface with the electrolyte layer.

It is not entirely clear to the inventors how the presence of thechemically unbound material on the negative electrode side in accordancewith the invention is acting to reduce degradation of the cellperformance, but it is clear that it does.

It is possible that the free or chemically unbound material acts as ascavenger to prevent or alleviate the reactions of negative electrodematerial poisons with the negative electrode material, that is elementsor species that react with the negative electrode material in some way,or at the electrode/electrolyte interface, causing reaction products inuse of the device that degrade the electrochemical performance of thecell. For convenience, therefore, and without limiting the scope of theinvention, the chemically unbound material may be referred to as“scavenger material” herein. The poisons may include sulphur species,boron species and phosphorus species, but other species such aschlorine, siloxane and selenium may also be reacting with or otherwiseaffecting the negative electrode material, or at theelectrode/electrolyte interface, and detrimentally affecting theperformance of the cell in use. The poison effect may include adsorptioninto an element or component of the electrode material, alloying with anelement or component of the electrode material, and forming salts (inthe case of, for example, chlorine) with an element or component of theelectrode material. The poisons may alternatively, or in addition, bedepositing at and adjacent the interface and blocking the fuel reactionsites. The poisons may be derived from the atmosphere, such as fuel inthe case of a fuel cell or steam in the case of a generator, or fromsystem components in or external to the negative electrode-side chamber.The free alkaline metal oxygen-containing compounds and/or free alkalineearth metal oxygen-containing compounds, usually in the form of oxidesbut not necessarily, have a higher chemical activity or affinity for thepoisons relative to the negative electrode material and therefore reactpreferentially with the poisons to prevent or alleviate the poisonsreaching at least the electrode/electrolyte interface, preferably toprevent or alleviate the poisons reaching or reacting with the negativeelectrode material in the functional layer at all. The element orcomponent of the negative electrode material with which the poisonswould be expected to react are the nickel or other metal in a cermetcomposition, but this may not always be the case.

It is possible that the chemically unbound material is also oralternatively acting in other ways to limit access of poisons to thenegative electrode material in some embodiments, such as by blocking therelease of negative electrode material poisons from the first gasseparator and/or other structure of the device, for example forming thenegative electrode-side chamber.

It is also possible that the chemically unbound material is acting insome other way to alleviate degradation of the electrochemicalperformance on the negative electrode side of the cell, includingincreasing electrical conductivity on the negative electrode side. Forexample, it is believed to be possible that the unbound material is insome way acting to alleviate sintering of the metal in a metal/cermetnegative electrode material.

It will be understood that the unbound material may be provided in or onany one or more of at least part of the first gas separator exposed tothe negative electrode-side chamber and any structure of theelectrochemical energy conversion device forming the chamber other thanthe negative electrode-side structure of the electrochemical cell andthe first gas separator, such as spacer plates, cell support plates,conductor layers and/or compliant layers, but in embodiments the unboundmaterial is provided in or on the negative electrode-side structure ofthe electrochemical cell.

Accordingly, in a second aspect of the invention there is provided anelectrochemical energy conversion cell comprising a layer of solid oxideelectrolyte, a negative electrode-side structure on one side of theelectrolyte layer and comprising one or more porous layers including afunctional layer of negative electrode material having an interface withthe one side of the electrolyte layer, and a positive electrode-sidestructure on the opposite side of the electrolyte layer and comprisingone or more porous layers including a layer of positive electrodematerial having an interface with the opposite side of the electrolytelayer, wherein chemically unbound material selected from one or both offree alkali metal oxygen-containing compounds and free alkaline earthmetal oxygen-containing compounds is provided in or on the negativeelectrode-side structure and acts to reduce degradation ofelectrochemical performance on the negative electrode side of theelectrochemical energy conversion cell during use of the cell, andwherein if the chemically unbound material is provided in the functionallayer of negative electrode material there is no chemically unboundmaterial present at the interface of that layer with the electrolytelayer.

The electrochemical energy conversion cell may take any form, such asplanar, tubular or monolithic, and the invention extends toelectrochemical energy conversion devices incorporating any of them.

The electrolyte layer may be a dense layer of any ceramic material thatconducts oxygen ions. Known ionic conductors that have been proposed assolid oxide electrochemical energy conversion cell electrolyte materialsinclude yttria-stabilised zirconia, often zirconia doped with 3 or 8 mol% yttria (3YSZ or 8 YSZ), scandia-stabilised zirconia, zirconia dopedwith 9 mol % Sc₂O₃ (9ScSZ), and gadolinium-doped ceria (GDC). The mostcommon of these is YSZ.

Some electrolyte layers may comprise sub-layers, for example a layer ofprimary electrolyte material and a surface layer of ionically conductiveceramic material. In one embodiment, destructive reactions can occurbetween YSZ electrolyte material and some positive electrode materialssuch as LSCF. It has been proposed to alleviate these by providing abarrier layer of ionically conductive YSZ-ceria on the electrolytematerial. In one embodiment the ceria may be doped with samarium. Onemethod of forming such a barrier layer is described in the applicant'sWO2010/040182.

It will be appreciated by those skilled in the art that the phrases“during use of the device”, “during use of the cell” and equivalentsencompass not only use of the device or cell to produce electricity oras an electrolyser, but also during pre-sintering of the cell and duringheating up of the device when the metal, usually nickel, of the negativeelectrode and other layers of the negative electrode-side structure maybe in the oxide state.

In embodiments, the chemically unbound material selected from one orboth of free alkali metal oxygen-containing compounds and free alkalineearth metal oxygen-containing compounds (hereinafter “chemically unboundmaterial” or “unbound material”) may be provided in an unbound materialcoating, which may be a surface coating (that is exposed at thesurface), on one or more of the negative electrode-side structure, thefirst gas separator and any other structure forming the negativeelectrode-side chamber, including any inlet to or outlet from thechamber. The unbound material coating may be applied as a continuous ordiscontinuous layer having, for example a thickness between 0.01 and 250μm, preferably between 0.01 and 50 μm. Although 0.01 μm has beenidentified as a minimum thickness, no specific minimum thickness hasbeen determined below which the coating becomes ineffective. However,thinner coatings become increasingly difficult to produce. A thicknessof 250 μm is believed to provide sufficient active material for manyyears of use of the electrochemical energy conversion device or cell,with a thickness of 50 μm providing sufficient protection on thenegative electrode side for the likely working life of the device orcell of up to 10 years. Greater thicknesses are possible, but in currentdesigns are considered unnecessary. The maximum thickness of the coatingmay also be dependent on the design of the negative electrode-sidechamber.

The unbound material coating may be continuous and dense on surfacesthrough which there is intended to be no gas transport and no electricalcontact, for example a non-electrically connecting gas separator and/orone or more other plates or components forming the negativeelectrode-side chamber. If the surface to which the coating is appliedis intended to have gases passing through it to or from the chamber, thecoating would be discontinuous, for example, porous and/or segmented.Thus, when formed on the outermost layer of the negative electrode-sidestructure of the electrochemical cell, generally a highly porous,electrically conducting, contact layer formed of nickel or otheracceptable metal, for example a noble or other metal that does notpoison the negative electrode material, including the metal of a cermetnegative electrode material, the unbound material coating woulddesirably be porous to permit gas flow through it. It may also oralternatively be segmented. If the porous coating is formed on thecontact face of a first gas separator in the form of an interconnect, itwould desirably be segmented so that it is not present at the contactpoints of the interconnect with the negative electrode-side structure orother conductive material. In some embodiments the interconnect haschannels, for example in the form of grooves, formed in the contactsurface for the transmission of gases to and/or from the negativeelectrode-side structure, in which case the coating material may only beprovided in the channels. Any coating of unbound material on the contactsurface must be sufficiently discontinuous to not unacceptable limit theelectrical contact.

The interconnect may have an electrically conductive contact layer onit, formed for example of nickel or other metal as above. Theinterconnect contact layer may have a thickness of 50 to 200 μm, forexample 75 to 150 μm, and is designed to ensure good thermal andelectrical contact with the negative electrode-side structure, but isadvantageously porous. If the coating of unbound material is alsoprovided, it may be beneath the contact layer of the interconnect or onit.

The first gas separator, when used as an interconnect, may have a densenickel, or other suitable conductive metal as above, coating on it toalleviate electrical resistance between the separator substratematerial, such as chromium-containing heat-resistant steel, and anadjacent layer of the separator or device, and the coating of unbound,material may be on this. The dense metal coating may have a thicknessof, for example, 10 to 100 μm, such as 15 to 50 μm. The dense metalcoating may be formed by spraying, for example thermal spraying.

Other layers may also or alternatively be provided on the first gasseparator. For example, at pages 5 and 6 of the aforementioned paper byFranco et al, it is noted that both diffusion of nickel can occur fromthe anode into a ferritic steel matrix substrate (that, can convert thesubstrate into an austenitic structure leading to mismatches in thecoefficient of thermal expansion) and diffusion of iron and chromiumspecies from the interconnect substrate into the anode up to theanode/electrolyte interface. The authors propose to alleviate this by adiffusion barrier layer or protective coating of perovskite materials,particularly doped LaCrO₃ perovskites. The unbound material coatingcould be provided on such a diffusion barrier layer.

Other structure that may be between the first gas separator and theelectrochemical cell in the negative electrode-side chamber includeseparate conductor and/or compliant layers, and these, or one or more ofthem, may also have the unbound material coating applied to them, or toother coating layers on them. The aforementioned compliant layer may be,for example, a metallic mesh such as of nickel or other suitableconductive metal as above. Two examples of nickel meshes, or nickelcoated, meshes, are described in the applicant's WO98/57384 andWO99/13522. The purpose of a compliant layer is to take up variations inthickness between the gas separator and the electrochemical cell whilebeing porous to atmosphere in the negative electrode-side chamber, aswell as to act as an electrical conductor. It may replace some or all ofthe contact layer(s).

The unbound material coating may consist only of the chemically unboundmaterial, for example free alkaline earth metal oxide or free alkalimetal oxide, possibly with some residual precursor oxygen-containingcompounds of the metal or metals such as nitrate or carbonate.

The unbound material coating may be formed by spray coating or otherwisecoating a solution of one or both of free alkali metal oxygen-containingcompound(s) and free alkaline earth metal oxygen-containing compound(s),or precursor material for it. Generally, the unbound material will befree oxide, which due to its reactivity can only be applied in precursorform. The compound(s) or precursor may be selected from salts such asnitrites, nitrates, carbonates, acetates and oxalates or fromhydroxides. The solution may be applied in plural passes in order toachieve the desired thickness but a single pass may be adequate. Aminimum single sprayed coating thickness may be 0.01 μm. Once applied tothe selected surface, the solution is dried, for example in an oven, ata drying temperature dependent upon the material. For strontium nitratethe drying temperature may be in the range 50-80° C.

The solution may comprise the oxygen-containing compound (which may be aprecursor) and water, optionally with a dispersant such, as2-amino-2-methyl-1-propynol. The dispersant is only required if there isa risk of the salt recrystallising in the solution and may be added at alevel sufficient to prevent recrystallisation, usually after thesolution has been formed. The water present in the coating solution willevaporate when the solution is dried. The oxygen-containing compound maybe added to the water at the maximum level that is readily dissolvable,which will vary for different compounds. The minimum level may bedependent upon the desired number of applications, such as spraycoatings. For strontium nitrate, the preferred concentration is between10 and 45 wt %, for example about 30 wt %, in water. Lowerconcentrations such as in the range 10 to 30 wt % tend to result infiner particles in the sprayed coating, leading to greater reactivity ofthe unbound material.

After drying, the scavenger coating is fired to burn off any dispersantand to convert any precursor scavenger material partially or totally tooxide. The minimum temperature at which this may be done is dependent onthe materials, but generally is about 450° C. for nitrate precursormaterials.

The firing may be performed during the manufacture of the component,after the coating has been applied, but is conveniently performed in theelectrochemical energy conversion device, during pre-sintering at atemperature in the range of, for example, 700° C. to 900° C., such as850° C.

In a variation of the unbound material where the unbound material orprecursor of it is not soluble in water, the coating may be screenprinted or otherwise applied, for example by spraying, as a slurry ofparticulate unbound material, or precursor, and binder which is thenfired. Suitable binders include those listed hereinafter for screenprinting inks. In embodiments, particle sizes are in the range of about0.01 to 25 μm, for example in the range 0.01 to 10 μm. Firing may beperformed as described above for the coating solutions.

Alternatively, or in addition to the unbound material coating, theunbound material may be present in any of one or more layers of thenegative electrode-side structure, one or more layers of the first gasseparator and one or more layers of any other structure of theelectrochemical energy conversion device, where each of those layers isaccessible to atmosphere in the negative electrode-side chamber. Whilethe unbound material may be localised in the respective layer, it ispreferably dispersed in it, at least throughout the portion of the layerexposed to the atmosphere in the negative electrode-side chamber.

Although in this embodiment the unbound material is provided in any ofthe one or more layers exposed to atmosphere in the negativeelectrode-side chamber, the unbound material remains free, that ischemically unbound to the chemical structure of the respective layer. Itmay therefore be more reactive than the layer material to poisons and/ormore active at preventing the release of poisons and/or more active atalleviating sintering of the negative electrode material, as describedabove.

The negative electrode-side structure of the electrolytic cell willinvariably comprise a layer of functional negative electrode materialwith a degree of porosity, generally a nickel cermet, possibly with amixture of two or more ceramic phases, of the type listed above adjacentthe layer of electrolyte material. The unbound material may be providedin the functional electrode layer, but advantageously there is none.Providing no unbound material in the functional layer of negativeelectrode material avoids any risk that the unbound material willdetrimentally impact on gas access through the porous functionalelectrode layer to the triple phase boundary of the gas with theelectrolyte and electrode materials. Such detrimental impact could be bythe unbound material physically blocking gas channels through the porousfunctional electrode layer and thereby restricting gas passage to orfrom the electrochemical reaction sites and/or by unbound materialphysically sitting on the reaction sites, making those reaction sitesinactive.

If unbound material is provided in the functional layer of negativeelectrode material, it may be dispersed evenly through the thickness ofthe layer of negative electrode material except at the interface of theelectrolyte and electrode materials. Providing no chemically unboundmaterial at the negative electrode/electrolyte interface is importantfor alleviating risk of the unbound material detrimentally impacting onthe reaction sites at the interface available to the fuel gas (in fuelcell mode) or for converting hydrogen ions to hydrogen (in electrolysermode) as described above. This may be done by grading the amount ofunbound material through the thickness of the functional electrodelayer, from a maximum at the surface remote from the electrolyte layerto zero at the interface. Alternatively, if the unbound material isprovided in the functional layer of negative electrode material, itmaybe provided only in a portion of the thickness of the layer remotefrom the interface. As the triple phase boundary area, that is the zonecontaining the reaction or active catalysing sites, may extend up toabout 10 microns in to the functional electrode layer from theinterface, advantageously that portion is at least 5 microns, preferablyat least 10 microns, more preferably at least 15 microns, from theinterface. In that portion of the thickness of the functional electrodelayer, the amount of unbound material may be even or graded.

If the negative electrode layer is not designed as a supporting layer,it may have a small thickness of, for example 5 to 50 μm, such as 10 to20 μm. If in addition to being a functional layer it is a supportinglayer, a thickness up to 250 μm or more may be required.

The negative electrode-side structure may include one or more porouslayers on the layer of functional negative electrode material. Theselayers must be porous to permit access of the atmosphere in the negativeelectrode-side chamber to the layer of negative electrode material. Theporosity in each of these layers may be about the same as that of thelayer of negative electrode material, but preferably it is greater toensure ready access of the atmosphere to the layer of negative electrodematerial.

If more than one porous layer is provided on the layer of negativeelectrode material, the porosity of all those layers may be the same,or, for example, it may increase for each layer more remote from thelayer of negative electrode material. The unbound material may beprovided in one or more of these layers, or in none of them. If it isprovided, it may be in each layer or in only one or some of plurallayers. In each layer in which it is provided, it may be localised orevenly dispersed through the thickness of the layer. Alternatively, itmay be graded through the thickness of the layer, increasing in amountaway from the layer of negative electrode material, or present in only aportion of the thickness of the layer, for example a portion most remotefrom the layer of positive electrode material.

An outermost porous layer of the negative electrode-side structure(excluding any unbound material or scavenger coating) may be a contactlayer, for example of nickel as described above, designed in the case ofthe gas separator being an interconnect to establish electrical contactbetween the cell and interconnect. It may have a thickness in the rangeof, for example, 20 to 100 μm and have a porosity in the range of, forexample, 10% to 85%. The thickness will generally depend upon the celland device design, but too thick a contact layer may lead to integrityproblems and cracking of the layer. Too thin a contact layer may lead totoo small a capacity to carry scavenger material in it. In oneembodiment the thickness may be in the range of 25 to 50 μm.

The contact layer may have even porosity throughout its thickness orincreasing porosity away from the layer of negative electrode material.One or more further contact layers may be provided between the outermostcontact layer and the layer of negative electrode material, preferablyeach such further contact layer having less porosity in theaforementioned range than the next adjacent contact layer on its sideremote from the layer of negative electrode material. Each such furthercontact layer may have even porosity throughout its thickness ofincreasing porosity from a side closest to the layer of negativeelectrode material.

Another porous layer that may be provided as part of the negativeelectrode-side structure is a substrate or support layer. The substratelayer will generally be disposed between the layer of negative electrodematerial and any contact layer and serves as a support layer for all ofthe other layers of the cell. It may be formed of the same or a similarcermet material as the negative electrode material, so that it performssome of the functions of the functional layer. The substrate layer mayhave a thickness of, for example, 100 to 500 μm, such as 150 to 250 μm.Too thin a substrate layer may not give it sufficient strength toperform its support function, but too great a thickness may lead toexcessive rigidity as well as greater resistance to gas transport andelectrical conductance.

In some SOFC designs, the physical support layer of the cell may be theelectrolyte layer or a positive electrode-side substrate layer, in whichcase the negative electrode-side substrate layer may be omitted and thefunctional layer of negative electrode material may be at or towards thelower end of the thickness range noted above.

The first gas separator may comprise any one or more of theaforementioned contact layer and barrier layer or protective coating ofperovskite or other material, and the unbound material may be providedin one or more of them.

As with any unbound material provided in the porous layers of thenegative electrode-side structure, the unbound material in any one ofthe aforementioned layers of the first gas separator exposed toatmosphere in the negative electrode-side chamber, may be localised orevenly dispersed throughout the layer. Alternatively it may be gradedthrough the thickness of the layer, for example increasing in amountaway from or towards the substrate gas separator material, or present inonly a portion of the thickness of the layer, such as a portion mostremote from the substrate gas separator material. Generally, however,the graded or localised provision is not necessary and the unboundmaterial is evenly dispersed in the respective layer.

One or more coating layers as described above, for example of nickel orsuitable other metal cermet or of metal alone, may be provided on otherstructure of the electrolytic energy conversion device forming thenegative electrode-side chamber, and unbound or scavenging material maybe provided in accordance with the invention in any one or more of thoselayers. Such other structure includes any inlet plenum component (whichmay be part of the first gas separator), a cover plate, a support plateand a compliant or conductive layer as already described. The coatinglayer on any of this structure may be dense or porous according to itsfunction. The coating layer differs from the aforementioned unboundmaterial coating in that the unbound material coating may compriseessentially only the chemically unbound material.

Generally, the layer or layers in which the unbound material may beprovided in accordance with this aspect of the invention are applied bytape casting an appropriate slurry composition and laminating the layersand/or screen printing an ink onto a substrate or previously-depositedlayer, This includes the functional layer of negative electrodematerial, any substrate layer of the negative electrode-side structure,any contact layer of the negative electrode-side structure, any contactlayer on the side of the first gas separator exposed to atmosphere inthe negative electrode-side chamber, and any such or other layer on anyother component of the electrochemical energy conversion device formingpart of the negative electrode-side chamber. Other methods, such as padprinting and spraying, may be used for forming each layer, but tapecasting is preferred for the anode functional and substrate layer andscreen printing is preferred for the contact layers and will bedescribed further.

The screen printing ink for each contact layer is formed by mixing theparticulate nickel, or other suitable metal, and a pore former withbinder, dispersant and solvent. The pore former may be omitted or atreduced levels for less porous layers. Suitable binders include alcoholsuch as ethanol or propanol mixed with ester such as hydroxypropylcellulose ether. Suitable dispersants include2-amino-2-methyl-1-propanol. The solvent may be a water miscibleorganic.

The tape casting slurries for the negative electrode functional andsubstrate layers may be formed by mixing particulate NiO or othersuitable metal oxide, YSZ or other suitable doped or stabilised oxide,and, in the case of the substrate layer, a pore former, with binder,dispersant and solvent as described above. If the negative electrodefunctional layer also acts as a substrate or support layer for theentire cell, some pore former may be added to ensure adequate porosity.

If the unbound material or its precursor dissolves in water, it may beadded to the base ink or slurry as a solution formed as described abovein relation to the unbound coatings. If the unbound material or itsprecursor is not dissolvable in water, it may be mixed into the ink orslurry as a powder or mixed with the base layer powder prior toformation of the slurry. Particle sizes for the unbound material may bein the range 0.01 to 25 μm, preferably in the range 0.01 to 10 μm. Themaximum particle size is limited by the thickness of the layer orcoating in which it is provided. Thus, for thicker coatings the unboundmaterial particle size may be greater than 25 microns. However, finerparticle sized unbound material will result in greater surface area,with a consequent increase in activity, The particle size of the unboundmaterial may be smaller than that of the layer material.

For the aforementioned barrier layer on the first gas separator, theunbound material may be provided in it by mixing the same solution orpowder, depending upon solubility, in the perovskite layer mixture.

The unbound material may be provided in any one porous layer, or in aprotective metal coating or barrier layer at a level in the range ofabout 0.1 to 65 vol % of the total solid content of the layer orcoating. More preferably, the range is about 1 to 25 vol %. In order tohave electrical continuity, the conducting phase should form at least 35vol % of the layer or coating, leaving a maximum of 65 vol % unboundmaterial. In practical terms, 35 vol % of the conducting phase providesrelatively low electrical conductivity, and a smaller proportion ofunbound material is preferred. While any proportion of unbound materialup to 65 vol % is acceptable, for example any proportion in the range 26to 65 vol %, the more preferred maximum is 25 vol % to provide a balancebetween good electrical conductivity and long term protection from theunbound material. While proportions of unbound material as low as 0.1vol % are believed to provide the desired protection, the more preferredminimum of 1 vol % will provide longer-term protection. In someembodiments, the proportion of unbound material relative to the totalsolid content, of the layer or coating is advantageously from 0.6 wt %,for example greater than 5 wt % or even 10 wt % or more.

Preferred cations for the unbound material are selected from one or moreof Sr²⁺, Ca²⁺, Ba²⁺, Mg²⁺, Na⁺ and K⁺.

As with the unbound material described with reference to unboundmaterial coatings, generally the chemically unbound material in one ormore of the aforementioned layers or coatings will comprise free oxide.However, due to its reactivity such a scavenging material can only beadded to the layer material in precursor form, preferably selected fromsalts such as nitrites, nitrates, carbonates, acetates and oxalates andfrom hydroxides. After tape casting the negative electrode functionaland/or substrate layer slurry with the precursor material in it, thelayer will be fired at a temperature in the range of 1300° C. to 1500°C. in air, resulting in the oxide being formed. At the same time, thebinder, dispersant and any graphite and residual solvent burn off,leaving the cermet layer, which in the case of the substrate layer isporous as a result of the graphite. Subsequently, the NiO reduces tonickel, resulting in some porosity in the functional layer. In avariation, the precursor material may be impregnated in to the layermaterial, as a solution, after firing the layer material. Impregnationis not preferred for at least the functional layer of negative electrodematerial of the negative electrode-side structure as it is difficult tocontrol the permeation of the solution through the layer(s), andtherefore difficult to ensure there is no unbound material at theinterface of the layer of electrolyte material with the functional layerof negative electrode material.

In the contact layers, the initial heating up of the stack in air to atemperature in the range of 700° C. to 900° C., during pre-firing and-sintering, result in the pore former, binder, dispersant and anyresidual solvent burning off to leave porous NiO (or other suitablemetal oxide) and any chemically unbound alkali metal oxide and alkalineearth oxide, as well as any residual free precursor material.Subsequently, the NiO reduces to nickel.

In many fuel cell systems for producing electricity, natural gas orother hydrocarbon is internally reformed in the stack, with the anodematerial, as the negative electrode of the cell acting as the reformingcatalyst. Under these circumstances it is desirable to only use freealkaline earth metal oxygen-containing compounds as the chemicallyunbound material since alkali metals tend to poison the reformingreaction.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of an electrochemical energy conversion device inaccordance with the invention and test results associated with thevarious embodiments will now be described by way of example only withreference to the accompanying drawings, in which:

FIG. 1 is a sectional view (not to scale) of a typical fuel cellassembly configuration of the type used to test the invention;

FIG. 2 is a graph of the effect on degradation of the cell assembly ofFIG. 1 of introducing free strontium oxide to a negative electrode sidecell contact layer;

FIG. 3 is a graph of the effect on degradation of the cell assembly ofFIG. 1 of introducing free strontium oxide to a positive electrode sidecell contact layer;

FIG. 4 is a graph of the effect on degradation of the cell assembly ofFIG. 1 of introducing free strontium oxide to a positive electrode sidecell contact layer as well as in both the contact layer and a shieldlayer;

FIG. 5 is a graph similar to FIGS. 2 to 4 but comparing the effect ofproviding strontium oxide in various locations on the positive electrodeside only, on the negative electrode side only, and on both electrodesides;

FIG. 6 is a graph similar to FIG. 5, but showing the effect ondegradation of providing strontium oxide on both positive and negativeelectrode sides over a longer period that in FIG. 5;

FIG. 7 is a graph of the effect on degradation of the cell assembly ofFIG. 1 of introducing free strontium oxide to individual locations ofthe assembly;

FIG. 8 is a graph similar to FIG. 7, but also showing the overall effecton degradation of providing strontium oxide on numerous locations of theassembly;

FIG. 9 is a graph showing the effect on degradation of different amountsof strontium oxide in various layers of the cell assembly;

FIG. 10 is a graph contrasting the effect on degradation of the cellassembly of FIG. 1 of introducing free oxide to various locations of thecell in different forms;

FIG. 11 is a graph showing the long-term effect on degradation of thecell assembly of FIG. 1 of introducing free strontium oxide toindividual locations of the assembly; and

FIG. 12 is a graph contrasting the effect on degradation of differentfree oxide precursors.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is sectional view illustrating a planar fuel cell assembly 10 ofthe type used to test the present invention. The figure is not to scaleand is provided in the form shown for ease of illustration. The assembly10 comprises a fuel cell unit 12 between opposed interconnect plates 14and 16. In a commercial fuel cell device, multiples of these assemblies10 would be stacked on top of each other, with each pair of abuttinginterconnector plates 14 and 16 being formed as a single plate. Asillustrated, the interconnect plate 14 is a positive electrode- orcathode-side interconnect, while the interconnect plate 16 is a negativeelectrode- or anode-side interconnect. They are formed ofchromium-containing high temperature resistant ferritic steel such asCrofer 22HJ Crofer 22APU and ZMG 232L.

Between the interconnect plates 14 and 16, the fuel cell unit 12comprises a dense electrolyte layer 18 of 8YSZ having a thickness in therange of 5 to 20 μm, for example 10 μm, with a doped ceria barrier layer20 on the cathode side. The barrier layer 20 prevents reactions betweenthe electrolyte layer 18 and certain cathode layers. Depending on thecombination of the electrolyte and cathode materials, the barrier layermay not be necessary. If it is provided, it may be mixed phase ceriazirconia layer as described in WO2010/040182 and may have a thickness inthe range of 0.5 to 1.5 μm.

The cathode layer 22 formed on the electrolyte barrier layer 20 is aporous perovskite such as LSCF and has a thickness in the range of 20 to30 μm. A cathode shield layer 24 is provided on the cathode layer 22,followed by a cathode cell contact layer 26, both formed of LSCo withthe contact layer being more porous than the shield layer, which may inturn be more porous than or of similar porosity to the cathode layer.The shield layer has a high degree of tortuosity relative to the contactlayer 26 and a thickness of about 20 μm and is designed with a largesurface area to reduce the likelihood of poisons o the cathode side ofthe fuel cell unit reaching the cathode layer 22 by virtue of thestrontium bound in the perovskite structure reacting with those poisonsin the pores of the shield layer. As will be appreciated, the shieldlayer 24 may be redundant in view of the provision of unbound scavengermaterial on the cathode side.

The cathode cell contact layer 26 has a thickness of about 125 μm andprovides an electrically conductive layer between the interconnect plate14 and the cathode layer 22.

The cathode interconnect plate 14 is provided with grooves or channels28 for gaseous oxidant, usually air, supply and the removal of gases onthe cathode side (in fuel cell mode). Between the grooves or channels 28peaks or lands 30 are defined which form contact faces with the cathodecell contact layer 26. To enhance the electrical contact between thecathode interconnect plate 14 and the cathode cell contact layer 26, acathode interconnect plate contact layer 32 is provided on the lands 30to directly contact the cathode cell contact layer 26. The cathode platecontact layer 32 is also formed of LSCo and has a similar porosity tothe contact layer 26. It may have a thickness of 75 to 125 μm.

Also on the cathode side, a barrier coating 34 is provided across theentire surface 36 of the interconnect plate 14 exposed to the oxidant inuse of the cell assembly, between the surface 36 and the contact layer32. The barrier coating 34 is intended to prevent the release ofchromium species from the interconnect plate 14, and may be a spinellayer as described in WO96/28855 having a thickness of 15 to 30 μm.

On the anode side, an anode functional layer 38 having a degree ofporosity is provided on the opposite side of the electrolyte layer 18 tothe cathode layer 22. It is formed of a Ni/8YSZ cermet having athickness of 10 to 12 μm.

A porous anode substrate layer 40 of Ni/3YSZ cermet having a thicknessof 180 to 200 μm is provided on the opposite side of the anodefunctional layer 38 to the electrolyte layer 18, and has a greaterporosity than the functional layer 38. The substrate layer 40 acts as astructural support layer for all of the other layers of the fuel cellunit 12.

An anode cell contact layer 42 is provided on the substrate layer 40 onthe opposite side to the functional layer 38 to enhance the electricalconnection between the anode substrate layer 40 and the anodeinterconnect plate 16. It is formed of porous metallic nickel, generallymore porous than the substrate layer 40, and has a thickness of about 30μm.

As on the cathode side, the anode interconnect plate 16 is provided withgrooves or channels 44 for the delivery of fuel gas to the anode side ofthe fuel cell unit 12 and removal of reacted fuel (in fuel cell mode).Between the grooves or channels 44, peaks or lands 46 are defined, andthe same porous nickel material is provided as an anode interconnectplate contact layer 48 on them. The contact layer 48 may have athickness of about 100 μm.

To improve electrical conductivity between the interconnect plate 16 andits contact layer 48, a dense layer of nickel 50 is formed on the side52 of the plate exposed to fuel. The dense nickel coating may have athickness of 15 to 45 μm and extends across the lands 46 and channels44.

Most if not all of these layers of the fuel cell assembly 10 are knownin the prior art and do not require describing further. However,briefly, the dense electrolyte layer 18 may be made by tape castingparticulate 8YSZ slurry and firing it. The electrolyte barrier layer 20is formed as described in WO2010/040182. The cathode layer 22 is formedby screen printing an ink made with LSCF perovskite material and hinder,onto the barrier layer 20 and firing it. The cathode shield layer 24 andcontact layer 26 are formed by screen printing an ink comprising LSCoperovskite material and binder, as well as a pore former such as carbon,polymer beads, corn starch, high molecular weight binders or graphite inthe case of the contact layer. The shield layer 24 is screen printedonto the cathode layer 22 and the contact layer 26 is screen printedonto the shield layer 24. After screen printing the layers are fired.

The cathode interconnect spinel barrier coating 34 may be formed asdescribed in WO96/28855, while the cathode interconnect contact layer 32is identical to the cathode cell contact layer 26 but screen printedonto the barrier coating 34. After screen printing the contact layer 32is fired.

The anode substrate layer 40 is formed first by tape casting a slurry ofNiO, 3YSZ, pore former selected from those described above, dispersantand solvent. During firing, the pore former, binder and dispersant burnoff, leaving pores and a substrate structure of NiO and 3YSZ. Afterpre-sintering of the assembly, the NiO in the substrate reduces to Ni toproduce the porous Ni/3YSZ substrate structure.

The anode functional layer 38 is tape cast as a slurry of NiO, 8YSZ,binder, dispersant and solvent. During firing, the binder, dispersantand solvent burn off leaving a smaller degree of porosity than in thesubstrate layer 40. After pre-sintering, the NiO in the substratereduces to Ni to provide the functional layer of Ni/8YSZ cermet.Porosity in the functional layer arises from the volume change occurringduring the NiO→Ni conversion. The electrolyte layer 18 is tape cast ontothe functional layer 38.

On the opposite side of the substrate layer 40 the anode cell contactlayer 42 is formed by screen printing an ink consisting of Ni, poreformer selected from those described above and binder. During theinitial heating of the cell assembly 10 (part of the pre-sinteringprocedure), the binder and pore former bum off leaving NiO which issubsequently reduced to porous Ni.

The tape east cell layers may be formed on a preceding layer or one ormore may be formed separately and laminated.

The same screen printing ink and procedure are used for the contactlayer 48 on the anode interconnect plate 16, while the dense contactlayer 50 is formed first by thermally spraying metal powder onto theface 52 so that it is formed in the groove 44 as well as on the lands46.

To complete the cell assembly 10 (prior to pre-sintering) the cathodeside of the assembly must be sealed from the anode side, and both mustbe sealed from external atmosphere. To do this, a series of glass seals54, 56 and 58 and a cover plate 60 are used. The various cathode sidelayers 22, 24 and 26 of the cell 12 do not extend to the edge of theelectrolyte layer 18, and the glass seal is formed on the electrolytebarrier layer 20 as an annulus that extends entirely around the cathodelayer 22. The cover plate 60 is formed of the same ferritic steel as theinterconnect plates 14 and 16 and is an annulus that is seated on theglass seal 54 and extends outwardly therefrom. Towards its outerperiphery 62, the cover plate 60 is also supported on the glass seal 56,which is itself an annulus that is supported on the anode interconnectplate 16 outwardly of the fuel cell unit 12 and at least the anodeinterconnect plate contact layer 48. The glass seal 58 is also anannulus that is supported on the cover plate 62 and extends to thecathode interconnect plate 14 outwardly of the cathode side of the fuelcell unit 12 and the cathode plate barrier coating 34.

A positive electrode or cathode side oxidant and exhaust chamber 64 isformed between the electrolyte barrier layer 20, the glass seal 54, thecover plate 60, the glass seal 58, the cathode interconnect plate 14and/or the cathode plate barrier coating 34, with the porous cathodelayer 22, cathode shield layer 24, cathode cell contact layer 26 andcathode side interconnect plate contact layer 32 being an integral partof the cathode side chamber 64.

Similarly, a negative electrode or anode side chamber 66 is formedbetween the electrolyte layer 18, the glass seal 54, the cover plate 62,the glass seal 56, the anode side interconnect plate 16 and/or the denseanode interconnect layer 50, with the anode functional layer 38, theanode porous substrate layer 40, the anode cell contact layer 42 and theanode interconnect plate contact layer 48 being an integral part of theanode side chamber 66.

It will be appreciated that at least one inlet to and at least oneoutlet from each chamber 64 and 66 must be provided to supply oxidant toand remove oxidant exhaust from the cathode side chamber 64 and supplyfuel gas and remove fuel exhaust from the fuel side chamber 66 (in fuelcell mode). These have not been shown in FIG. 1 merely for clarity ofthe section.

As described, the cell assembly 10 formed the basis of the testsrepresented as “no Sr” in FIGS. 2 to 8 and 10. It will be appreciatedfrom these graphs of electrical output degradation (measured aspercentage variation relative to an output at about 0 hours againsttime, where a positive degradation means the electrical output isdecreasing) that the cell assembly as described suffers from electricaloutput degradation for at least one of the reasons described herein.

It will be understood that this degradation occurs even though, forexample, at least the cathode shield layer 24 and the cathode platebarrier coating 34 as well as the anode plate dense contact layer 50 aredesigned to alleviate the degradation.

The applicant has found, though, that the provision of free alkalineearth metal oxygen containing compounds and/or free alkali metal oxygencontaining compounds on or in one or both of the positive and negativeelectrode-side chambers can substantially alleviate the electricalperformance degradation. If the fuel cell assembly is to be used forinternally reforming hydrocarbon fuel gas, such as natural gas, tohydrogen, alkali metal oxygen containing compounds should be avoided onthe negative electrode or anode side since these compounds tend to havea detrimental effect on the reforming reaction.

Generally the oxygen-containing compound will be an oxide, but due tothe reactivity of these oxides they need to be added as precursor.Depending on the thermal stability of the precursor, some precursormaterial may exist as free oxygen-containing compound alongside the freeor unbound oxide. The preferred oxide is SrO (conveniently referred toas “Sr” in FIGS. 2 to 9), but other oxides in the two Groups are knownto perform very similarly, particularly CaO, BaO, MgO, Na₂O and K₂O.

The oxygen-containing compound can be provided in or on any one ofvarious components of one or both of the cathode and anode chambers 64and 66. FIGS. 2 to 9 and 11 illustrate the provision of free SrO, andany residual strontium nitrate precursor, in various of those locationsidentified in the Figures as positions 1-7. The location of thesepositions relative to the cell assembly 10 of FIG. 1 is identified inTable 1.

FIG. 10 also illustrates the provision of SrO from strontium nitrateprecursor at positions 2 to 6 and the description below for thisprovision in relation to FIGS. 2 to 9 applies to FIG. 10 also. However,FIG. 10 contrasts this provision with providing the free SrO fromstrontium carbonate and free CaO from calcium carbonate, each at thesame locations, positions 2 to 6.

FIG. 12 contrasts the effect on degradation of using different strontiumand calcium precursor materials and forms.

TABLE 1 SrO Position Description of Location 1 Free SrO coating on theanode interconnect plate 16 between the dense contact layer 50 and theporous nickel contact layer 48. 2 Free SrO dispersed in the anodeinterconnect plate porous contact layer 48. 3 Free SrO dispersed in theanode cell porous nickel contact layer 42. 4 Free SrO dispersed in thecathode shield layer 24. 5 Free SrO dispersed in the cathode cell porouscontact layer 26. 6 Free SrO dispersed in the cathode interconnectcontact layer 32. 7 Free SrO coating on the cathode interconnect plate14 between the cathode interconnect plate barrier coating 34 and thecathode interconnect plate porous contact layer 32.

In positions 1 and 7, the free SrO coating or wash coat is applied tothe entire extent of the respective dense layer 50 and 34, including inthe respective grooves or channels 44 and 28, not just beneath therespective interconnect plate porous contact layer 48 or 32.

The same strontium solution is used for both the coatings at positions 1and 7 and for dispersing the strontium nitrate precursor in the layersat positions 2 to 6. The Sr(NO₃)₂ is converted to SrO on heating in air.

The strontium solution is made up of strontium nitrate, water and adispersant. Initially, strontium nitrate is weighed, followed by theaddition of a required amount of water. The strontium nitrate isdissolved in the water by heating the mixture in a water bath in a:

-   -   temperature range of 40-70° C., while being stirred. A        dispersant is added to the solution to prevent the strontium        nitrate from recrystallising at temperatures less than 15° C.        and to assist dispersing the strontium nitrate in the inks used        for incorporating the strontium into the layer materials for        positions 2 to 6. The concentration of strontium nitrate        solution is slightly lower than the saturation level at normal        temperature and pressure, to avoid the recrystallisation        problem.

For the porous nickel contact layers 48 and 42 at positions 2 and 3, thestrontium is dispersed in the following way. The required quantities ofnickel powder, pore former and binder are weighed and mixed in a highshear mixer. Once the mixture is homogenised, the required quantity ofthe strontium nitrate solution is added and the new mixture ishomogenised again in the high shear mixer to produce an ink suitable forscreen printing of the layers.

For the porous perovskite contact layers 26 and 32 at positions 5 and 6,the required quantities of lanthanum strontium cobaltite (LSCo) powderand binder are mixed by hand until they are blended together. Theblended mixture is then triple roll milled for a number of passes beforethe pore former and additional binder are added to the triple-rolledmixture and homogenised in a high shear mixer. Once the mixture ishomogenised, the required quantity of the strontium nitrate solution isadded and homogenised again in the high shear mixer to produce an inksuitable for screen printing of the layers.

The screen printing ink preparation for the perovskite cathode shieldlayer 24 at position 4 is prepared in exactly the same way as thecathode side porous contact layer inks, except that no pore former isadded to the mixture.

For the screen printing inks, the strontium may be added to a levelwhere in the fired product the free strontium oxide and any residualprecursor strontium nitrate are present at a total level of from 0.1 to65 vol % relative to the total solids content of the layer, morepreferably from 1 to 25 vol %. While there may be advantages toproviding levels of the free strontium material above 25 vol % up to themaximum indicated of 65 vol %, doing so may lead to difficulties inmaintaining the stability of the screen printing inks, and it is forthis reason that 25 vol % is the preferred maximum. Levels of strontiumoxide tested in the cathode side contact layers and shield layer havebeen from 2.8-13.2 wt %, while the corresponding range for the anodeside contact layers is 0.6-13.2 wt %. Free strontium nitrate has alsobeen added to the anode substrate layer 40, at a level of 0.64 wt %.

Table 2 sets out Examples of compositions for the strontium nitratesolution, the cathode side porous contact layer inks, the cathode shieldlayer ink and the anode side porous contact layer inks. In Table 2, noranges are given for the amount of LSCo, pore former and nickel as theLSCo/pore former and Ni/pore former ratios were maintained constant.

Some commercial names are referred to in Table 2 (and in Tables 6 to 8),and these are explained in Table 3, along with their source.

The strontium nitrate solution is applied to the cathode and anodeinterconnect plates 14 and 16 by spraying the solution onto therespective faces 36 and 52, over the respective dense layers 34 and 50,but excluding the areas contacted by the glass seals 58 and 56,respectively. The required weight of strontium nitrate is achieved bycontrolling the number of spray passes, to provide coating thicknessesin the range 0.01 to 250 μm, preferably 0.01 to 50 μm.

TABLE 2 Sr(NO₃)₂ Solution Range Current with min. with max. MaterialWeight, g AMP95 AMP95 Scavenger precursor Sr(NO₃)₂ 36 36.0% 36.0% MediumWater 54.8 64.0% 44.0% Dispersant AMP95 9.2 0.0% 20.0% Current PreferredBroadest Material Weight, g Range, g Range, g Positive side Porous Layerink Conducting LSCo 32.9 32.9 32.9 Phase Binder 1 Cerdec 80683 18.710-25 1-25 Binder 2 Cerdec 80858 18.7 10-25 1-25 Pore-former Graphite16.0 16.0 16.0 Scavenger Sr(NO₃)₂ solution 13.7 1.32-42.0 0.132-232  Positive side Shield Layer ink Conducting LSCo 48.9 48.9 48.9 PhaseBinder 1 Cerdec 80683 16.3 10-25 1-25 Binder 2 Cerdec 80858 16.3 10-251-25 Scavenger Sr(NO₃)₂ 18.5 1.97-62.4 0.197-345   solution Negativeside Porous Layer ink Conducting Ni 35.6 35.6 35.6 Phase Binder 1 Cerdec80683 18.3 10-25 1-25 Binder 2 Cerdec 80858 18.3 10-25 1-25 Binder 3PreGel 5.3  5.30  5.30 Pore-former Graphite 15.3 15.3 15.3 ScavengerSr(NO₃)₂ 7.2 1.43-45.5 0.143-251   solution

TABLE 3 Commercial Name Chemical wt % Manufacturer AMP952-Amino-2-methyl-1-Propanol 95% Angus Chemical Company, Water 5% USACerdec 80858 Propanol ≧60% Ferro Corporation, USA Hydroxy propylcellulose ether Cerdec 80683 Ethanol, & Ferro Corporation, USA2-(2-ethoxyethoxy)-ethanol Hydroxy propyl cellulose ether PreGel Cerdec80858 46% Ceramic Fuel Cells Ltd, Cerdec 80683 46% Australia CrayvallacSuper 8% Crayvallac MicronisedPolyamide wax Arkrma, France Super DGMEDiethylene GlycolMonoethyl ether DOW Chemicals, USA LSCo LanthanumStrontium Cobalt Oxide Fuel Cell Materials, USA Nickel Ni NovametSpecialty Products Corporation, USA

Referring now to the graphs, in FIG. 2 free strontium oxide is providedonly in the anode cell contact layer 42 of the cell assembly 10 andprovides a substantial reduction in the degradation in electrical outputfrom the assembly over the First 2000 hours of operation relative to nofree SrO.

Referring to FIG. 3, the free strontium oxide is provided in the cellassembly 10 only in the cathode cell contact layer 26, and may be seento reduce the electrical output degradation from about 5% (when no freeSrO is present) at nearly 2000 hours to under 2%.

FIG. 4 duplicates the results of FIG. 3, but also shows the effect ofadditionally including free strontium oxide dispersed in the cathodeshield layer 24. With the free strontium oxide dispersed in both thecathode cell contact layer 26 and the cathode shield layer 24, theelectrical output degradation is limited to no more than 1% over nearly2000 hours.

In FIG. 5, the electrical output degradation of the cell assembly 10with no free strontium oxide present is contrasted over 2500 hours withproviding free strontium oxide at positions 4, 5, 6 and 7 on the cathodeside only, at positions 1, 2 and 3 on the anode side only, at positions2, 3, 4, 5, and 6 (so no free strontium oxide interconnect platecoatings on either side), and at all of positions 1 to 7. It may be seenthat over this time frame, the cell electrical output degradationdecreases from about 6.5% with no strontium oxide present to about 4.8%with free strontium oxide only present on the cathode side, to about1.3% for each of the tests in which free strontium oxide is only presenton the anode side and is dispersed in all four contact layers and In theshield layer, to about 0.1% when free strontium oxide is present in all7 positions.

FIG. 6 illustrates the results of another test of the cell assembly 10in which free strontium oxide is provided at all 7 positions. Over about3700 hours, the cell electrical output degradation remained less than1%. In contrast, at least by 3000 hours the test showed that with nofree strontium oxide present the cell electrical output degradation wasgreater than 5%.

FIG. 7 illustrates another short term test of the cell assembly 10, overabout 800 hours, contrasting the provision of no free strontium oxide inthe assembly with individual tests where free strontium oxide isprovided at positions 1, 2, 3, 4, and 5, respectively. It may be seenthat in this test, the provision of free strontium oxide at position 1only, as a coating on the anode side interconnect plate, improved theelectrical output over the initial 450 hours, with negligibledegradation in output thereafter.

FIG. 8 is identical to FIG. 7 except that it also shows the results of atest in which free strontium oxide was provided at all of positions 1 to7. In this test, the final outcome at about 800 hours for providing thefree strontium oxide at all 7 positions was a cell output degradation ofabout 0.8%, worse than when it was provided only at position 1. Thereason for this is not clear to the inventors.

FIG. 9 shows the results of a test of a stack with 7 layers of cellassembly 10 with different proportions of free strontium oxide presentin 3 layers of the cell 12, the anode contact layer 42 (position 3), thecathode shield layer 24 (position 4) and the cathode contact layer 26(position 5). The wt % of free strontium oxide in each layer isidentified in the Figure by the formula: Layer—#: A/B/C wherelayer—#—the layer number; A=the SrOwt % in the anode contact layer;B=the SrOwt % in the cathode shield layer; and C=the SrOwt % in thecathode contact layer.

It may be seen from FIG. 9 that all of the layers showed a cellelectrical output degradation of less than 1% over the 500 hours tested,but that layers 1, 3, 4 and 7 all showed an improvement in electricaloutput. Layer 2 showed overall no change in electrical output after 500hours, while layers 5 and 6 (with no free strontium oxide in the anodecontact layer) showed higher cell output degradations at 500 hours of0.8% and 0.5 respectively. These may be contrasted with the results inFIGS. 7 and 8, where with no free strontium oxide present at all in thecell assembly the cell output degradation over a similar period wasabout 2% and over 800 hours approached 4%.

FIG. 10 illustrates another short term test of the cell assembly 10,over about 340 hours, contrasting the provision of no free strontiumoxide in the assembly with individual tests where free oxide is providedat positions 2, 3, 4, 5, and 6, respectively. In this test, the freeoxide is in three different forms, strontium oxide from strontiumnitrate precursor, strontium oxide from strontium carbonate precursorand calcium oxide from calcium carbonate precursor. The strontiumnitrate is introduced in the manner described above for positions 2 to6. The strontium carbonate and calcium carbonate are also added inexactly the same way using the compositions set out in Table 4 and 5,respectively, to achieve the same level of active metal content as inthe strontium nitrate.

It may be seen from FIG. 10 that in this test the cell electrical outputdegraded by about 1.6% over the 340 hours of the test when no free oxideis added, while the output from the cells with strontium oxide fromstrontium nitrate and calcium oxide from calcium carbonate in positions2 to 6 degraded by about 0.2% over the same period. On the other hand,the provision of strontium oxide from strontium carbonate at positions 2to 6 improved the cell output over the same period by about 0.5%.

TABLE 4 FORMULATIONS WITH SrCO₃ Material Current Weight, g Positive sidePorous Layer ink Conducting Phase LSCo 31.6 Binder 1 Cerdec 80683 20.2Binder 2 Cerdec 80858 20.2 Solvent DGME 9.5 Pore-former Graphite 15.2Scavenger SrCO₃ 3.3 Positive side Shield Layer ink Conducting Phase LSCo49.6 Binder 1 Cerdec 80683 20.8 Binder 2 Cerdec 80858 20.8 Solvent DGME3.6 Scavenger SrCO₃ 5.1 Negative side Porous Layer ink Conducting PhaseNi 38.9 Binder 1 Cerdec 80683 18.2 Binder 2 Cerdec 80858 18.2 SolventDGME 6.1 Pore-former Graphite 16.6 Scavenger SrCO₃ 2.0

TABLE 5 FORMULATIONS WITH CaCO₃ Material Current Weight, g Positive sidePorous Layer ink Conducting Phase LSCo 26.9 Binder 1 Cerdec 80683 29Binder 2 Cerdec 80858 29 Pore-former Graphite 12.9 Scavenger CaCO₃ 2.2Positive side Shield Layer ink Conducting Phase LSCo 47.2 Binder 1Cerdec 80683 21.9 Binder 2 Cerdec 80858 21.9 Solvent DGME 5.0 ScavengerCaCO₃ 4.0 Negative side Porous Layer ink Conducting Phase Ni 41.9 Binder1 Cerdec 80683 15.5 Binder 2 Cerdec 80858 15.5 Solvent DGME 6.1Pore-former Graphite 18.0 Scavenger CaCO₃ 3.0

FIG. 11 contrasts the effect on output degradation over 12,000 hours ofthe provision of strontium oxide derived from strontium nitrate solutionin one or more of positions 3, 4 and 5 in a stack comprising 51 cellssimilar to that of FIG. 1 with output degradation over the same periodwhen no free oxide scavenger material is provided. This test iseffectively an extension of the test described with reference to FIG. 9,with the layer materials for the anode contact layer (position 3), theshield layer (position 4) and the cathode contact layer (position 5)being prepared as described with reference to Table 2.

The test showed that over the 12,000 hours the cell with no free oxidepresent suffered degradation in electrical output of 10.6%. On the otherhand, with free strontium oxide at various of positions 3, 4 and 5 thedegradation over the same period was 6.2% at position 3 only, 5.1% atposition 5 only, 4.7% at positions 4 and 5, and 3.9% at positions 3, 4and 5. These degradation rates are the average of several cell layershaving the same configuration.

This shows that over the test period of 500 days the provision of freeoxide on each of the cathode and anode sides of the cell significantlyreduced the degradation in electrical output of the cell—by well over50% with the free oxide at position 5 only, increasing by about another4% when the free oxide is both positions 4 and 5 on the cathode side,and by about 45% with the free oxide at position 3 on the anode sideonly. Furthermore, when the free oxide is provided at all 3 of positions3, 4 and 5 on the anode and cathode sides the degradation was reduced byalmost two thirds over the 500 days.

FIG. 12 contrasts the effect on cell output degradation over 2,000 hoursof the provision of free oxide derived from different precursors withoutput degradation over the same period when no free oxide scavengermaterial is provided.

Four different precursor materials and forms were tested: Sr(NO₃)₂solution; CaCO₃ powder; Sr(NO₃)₂ powder; and SrCO₃ powder. Theprecursors were tested in the same positions in respective cells of astack, namely positions 3, 4 and 5. The layers comprising the Sr(NO₃)₂were prepared as described above with reference to Table 2, while eachlayer comprising one of the powders was prepared according to Table 6, 7or 8, respectively.

The test showed that over the 2,000 hours the cell with no oxide presentsuffered degradation in electrical output of 4.3%. On the other hand,the presence of free oxide derived from the different precursors reducedthe degradation to the following levels over the same period: 2.7% forSr(NO₃)₂ solution; 2.6% for Sr(NO₃)₂ powder; 2.5% for SrCO₃ powder; and1.7% for CaCO₃ powder. Thus, it may be seen that, for example, theprovision of free oxide derived from CaCO₃ as described reduced the celloutput degradation by about 60% or 2.5 times over the test period.

TABLE 6 Formulations with CaCO₃ Powder Material Current Weight, gCathode Contact Layer Conducting Phase LSCo 32.68 Binder 1 Cerdec 8068322.87 Binder 2 Cerdec 80858 22.87 Solvent DGME 3.4 Pore-former Graphite15.87 Scavenger CaCO₃ 2.31 Shield Layer Conducting Phase LSCo 51.96Binder 1 Cerdec 80683 18.6 Binder 2 Cerdec 80858 18.6 Solvent DGME 7.5Scavenger CaCO₃ 3.34 Anode Contact Layer Conducting Phase Ni 35.46Binder 1 Cerdec 80683 17.41 Binder 2 Cerdec 80858 17.41 Solvent DGME7.68 Binder 3 PreGel 5.15 Pore-former Graphite 15.23 Scavenger CaCO₃1.65

TABLE 7 Formulations with Sr(NO₃)₂ Powder Material Current Weight, gCathode Contact Layer Conducting Phase LSCo 31.89 Binder 1 Cerdec 8068322.32 Binder 2 Cerdec 80858 22.32 Solvent DGME 3.17 Pore-former Graphite15.51 Scavenger Sr(NO₃)₂ 4.78 Shield Layer Conducting Phase LSCo 49.16Binder 1 Cerdec 80683 17.60 Binder 2 Cerdec 80858 17.60 Solvent DGME9.00 Scavenger Sr(NO₃)₂ 6.65 Anode Contact Layer Conducting Phase Ni35.65 Binder 1 Cerdec 80683 17.50 Binder 2 Cerdec 80858 17.50 SolventDGME 6.44 Binder 3 PreGel 5.14 Pore-former Graphite 15.34 ScavengerSr(NO₃)₂ 2.44

TABLE 8 Formulations with SrCO₃ Powder Material Current Weight, gPositive side Porous Layer ink Conducting Phase LSCo 32.13 Binder 1Cerdec 80683 22.49 Binder 2 Cerdec 80858 22.49 Solvent DGME 3.94Pore-former Graphite 15.60 Scavenger SrCO₃ 3.35 Positive side ShieldLayer ink Conducting Phase LSCo 51.40 Binder 1 Cerdec 80683 18.40 Binder2 Cerdec 80858 18.40 Solvent DGME 6.97 Scavenger SrCO₃ 4.83 Negativeside Porous Layer ink Conducting Phase Ni 36.04 Binder 1 Cerdec 8068317.70 Binder 2 Cerdec 80858 17.70 Solvent DGME 6.12 Binder 3 PreGel 5.24Pore-former Graphite 15.49 Scavenger SrCO₃ 1.72

Further tests were conducted on the cathode side to assess the abilityof free oxides derived from different precursors to absorb chromiumemissions from a spinel coated interconnect plate prepared as describedwith reference to FIG. 1. Chromium is a major poison of SOFC cathodematerials, and the ability of the free oxide to scavenge such emissionsfrom the interconnect plate or elsewhere rather than them reacting withthe cathode material is an important consideration.

In the first test, the abilities of free oxides derived from differentcarbonate precursors and an LSCF cathode material to absorb the chromiumemissions from the spinel coated interconnect plate were investigated at800° C. over a period of 50 hours. The alkaline earth metal carbonatesalts CaCO₃, BaCO₃ and SrCO₃ were each milled to a particle sizetypically less than 2 μm and were then turned into inks suitable forscreen printing use. The ink formulas were similar to the Positive SideShield Layer Inks described in Tables 4 and 5, with carbonate saltsfully replacing LSCo). The LSCF cathode material powder was turned intoan ink in a similar manner. All inks were screen printed on to a 3YSZsubstrate wafer, forming a coating layer approximately 45 μm thick. The3YSZ substrate was about 100 μm thick. The LSCF coating was subjected toa firing cycle typical of normal cathode layer fabrication tiring asdescribed herein and all the carbonate coatings were dried at 70° C.only to prepare all the coatings for chromium emission testing.

For the chromium emission test, the 3YSZ wafers with various coatingmaterials were broken into small pieces approximately 10 mm×20 mm insize. These small coupons were placed on top of the spinel coatedinterconnect plate, with the coating materials facing the plate. Theinterconnect plate, with coated 3YSZ coupons sitting on top, was firedin atmospheric air and allowed to cool. Once cooled, the coatingmaterials were removed from the 3YSZ substrate by dissolving into anacid solution (usually hydrochloric acid), and analysed for chromiumcontent.

The results are given in Table 9 and show that free oxide derived fromeach of CaCO₃, BaCO₃ and SrCO₃ has a far greater ability to absorb thechromium emissions than the cathode material and therefore that thesefree oxide materials in or on various layers of the cathode-side chamberof a device such as is shown in FIG. 1 will be effective in scavengingthe chromium emissions before they are able to reach thecathode/electrolyte interface.

TABLE 9 Material |Cr| · ppm CaCO₃ 8440 BaCO₃ 6577 SrCO₃ 3140 LSCF 134

In the second test, the abilities of free oxides derived from two otherprecursor materials, SrC₂O₄ and NaOH, were tested in comparison with twoLSCF cathode materials. One LSCF material was from an earlier purchasebatch, applied on a 3YSZ substrate as a coating, designated as LSCFcoated 3Y—ZrO₂. The other was from a more recent batch, applied on ananode-supported 8YSZ electrolyte substrate (with a spinel barrier layeras described above) as a coating, and designated as LSCF cathode halfcell. Both LSCF coatings were applied by screen printing followed by asinter firing typical for LSCF cathode fabrication. The first precursormaterial SrC₂O₄ was provided as an ethanol based SrC₂O₄ slurryimpregnated into the porous LSCF layer of the LSCF cathode half cell,and the second precursor material NaOH was provided as a 0.5M NaOHaqueous solution infiltrated into the porous LSCF layer of the LSCFcathode half cell. Both LSCF coatings, as well as the SrC₂O₄ slurryimpregnated LSCF cathode half cell and the NaOH solution infiltratedLSCF cathode half cell, were tested to absorb chromium emission from thespinel coated interconnect plate at 650° C. over a period of 20 hours.The chromium emission test set-up was similar to that described for thefirst test.

The test was run twice and the results are given in Table 10. They showthat the free oxide derived from each of SrC₂O₄ and NaOH has a greaterability to absorb the chromium emissions than the cathode material andtherefore that these free oxide materials in or on various layers of thecathode-side structure of a fuel cell or electrolyser such as is shownin FIG. 1 will be effective in scavenging the chromium emissions beforethey are able to reach the cathode/electrolyte interface.

TABLE 10 Structure |Cr| · ppm-run 1 |Cr| · ppm-run 2 LSCF coated 3YSZ 8486 LSCF cathode half cell 194 196 Half cell + SrC₂O₄ slurry 584 406 Halfcell + NaOH solution 256 206

The test results for the absorption of chromium species by the half cellwith the NaOH solution coating are not substantially better than thosefor the half cell alone, especially in the second run, but this is as aresult of using a weak hydroxide solution. It is believed that using astronger solution will produce significantly better results.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is to be understood that the inventionincludes all such variations and modifications which fall within itsspirit and scope.

Whilst the present invention has been described with reference tospecific embodiments and planar fuel cells, it will be appreciated thatsuch embodiments are merely exemplary, and other embodiments other thanthose described herein will be encompassed by the invention as definedby the appended claims.

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that that prior publication (or information derived from it)or known matter forms part of the common general knowledge in the fieldof endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

1. An electrochemical energy conversion device comprising a stack ofsolid oxide electrochemical cells alternating with gas separators,wherein each electrochemical cell comprises a layer of solid oxideelectrolyte, a negative electrode-side structure on one side of theelectrolyte layer and comprising one or more porous layers including afunctional layer of negative electrode material having an interface withthe one side of the electrolyte layer, and a positive electrode-sidestructure on the opposite side of the electrolyte layer and comprisingone or more porous layers including a layer of positive electrodematerial having an interface with the opposite side of the electrolytelayer, wherein said electrochemical cell and a first of the gasseparators on the negative electrode side of the electrochemical cell atleast partly form therebetween a negative electrode-side chamber andsaid electrochemical cell and a second of the gas separators on thepositive electrode side of the electrochemical cell at least partly formtherebetween a positive electrode-side chamber, and wherein chemicallyunbound material selected from one or both of free alkali metaloxygen-containing compounds and free alkaline earth metaloxygen-containing compounds is provided in or on one or more of thenegative electrode-side structure, the first gas separator and any otherstructure of the electrochemical energy conversion device forming thenegative electrode-side chamber, the chemically unbound material actingto reduce degradation of electrochemical performance on the negativeelectrode side of the electrochemical energy conversion device duringuse of the device, and wherein if the chemically unbound material isprovided in the functional layer of negative electrode material there isno chemically unbound material present at the interface of that layerwith the electrolyte layer.
 2. An electrochemical energy conversiondevice according to claim 1, wherein the chemically unbound material isa scavenger material that is accessible to negative electrode poisons inthe atmosphere in the negative electrode-side chamber during use of thedevice and is more reactive with the poison than is the negativeelectrode material.
 3. An electrochemical energy conversion deviceaccording to claim 1, wherein the chemically unbound material isprovided in a chemically unbound material coating on one or more of thenegative electrode-side structure, the first gas separator and said anyother structure forming the negative electrode-side chamber.
 4. Anelectrochemical energy conversion device according to claim 3, whereinsaid any other structure forming the negative electrode-side chambercomprises one or more of a separate conductor layer and a separatecompliant layer between the first gas separator and the negativeelectrode-side structure.
 5. An electrochemical energy conversion deviceaccording to claim 3, wherein the chemically unbound material coating isdiscontinuous.
 6. An electrochemical energy conversion device accordingto claim 3, wherein the chemically unbound material coating has athickness of about 0.01 to 250 μm.
 7. (canceled)
 8. An electrochemicalenergy conversion device according to claim 1, wherein the chemicallyunbound material is dispersed in at least one of the one or more porouslayers of the negative electrode-side structure.
 9. An electrochemicalenergy conversion device according to claim 8, wherein the one or moreporous layers of the negative electrode-side structure comprises, inaddition to the functional layer of negative electrode material, anegative electrode-side layer of electrical contact material. 10.(canceled)
 11. An electrochemical energy conversion device according toclaim 8, wherein the one or more porous layers of the negativeelectrode-side structure comprises, in addition to the functional layerof negative electrode material, a negative electrode-side layer ofsubstrate material.
 12. (canceled)
 13. An electrochemical energyconversion device according to claim 1, wherein the first gas separatorcomprises a dense substrate, one or more porous layers on a side of thesubstrate facing the negative electrode-side chamber and/or a protectivecoating on a side of the substrate facing the negative electrode-sidechamber and in contact with the substrate, and wherein the chemicallyunbound material is provided in at least one of the one or more porouslayers of the first gas separator and/or the protective coating. 14.(canceled)
 15. (canceled)
 16. An electrochemical energy conversiondevice according to claim 13, wherein the chemically unbound material inany one porous layer or in the protective coating is provided at a levelin the range of about 0.1 to 65 vol % of the total solid content of thelayer or coating.
 17. (canceled)
 18. A electrochemical energy conversiondevice according to claim 1, wherein the chemically unbound materialcomprises free oxide selected from one or more of SrO, CaO, BaO, MgO,Na₂O and K₂O.
 19. An electrochemical energy conversion cell comprising alayer of solid oxide electrolyte, a negative electrode-side structure onone side of the electrolyte layer and comprising one or more porouslayers including a functional layer of negative electrode materialhaving an interface with the one side of the electrolyte layer, andpositive electrode-side structure on the opposite side of theelectrolyte layer and comprising one or more porous layers including alayer of positive electrode material having an interface with theopposite side of the electrolyte layer, wherein chemically unboundmaterial selected from one or both of free alkali metaloxygen-containing compounds and free alkaline earth metaloxygen-containing compounds is provided in or on the negativeelectrode-side structure and acts to reduce degradation ofelectrochemical performance on the negative electrodeside of theelectrochemical energy conversion cell during use of the cell, andwherein if the chemically unbound material is provided in the functionallayer of negative electrode material there is no chemically unboundmaterial present at the interface of that layer with the electrolytelayer.
 20. An electrochemical energy conversion cell according to claim19, wherein the chemically unbound material is a scavenger material thatis accessible to negative electrode poisons in atmosphere contacting thenegative electrode-side structure during use of the cell and is morereactive with the poisons than is the negative electrode material. 21.An electrochemical energy conversion cell according to claim 19, whereinthe chemically unbound material is provided in a discontinuous unboundmaterial coating on the negative electrode-side structure.
 22. Anelectrochemical energy conversion cell according to claim 21, whereinthe chemically unbound material coating has a thickness of about 0.01 to250 μm.
 23. (canceled)
 24. An electrochemical energy conversion cellaccording to claim 19, wherein the chemically unbound material isdispersed in at least one of the one or more porous layers of thenegative electrode-side structure.
 25. An electrochemical energyconversion cell according to claim 24, wherein the one or more porouslayers of the negative electrode-side structure comprises, in additionto the functional layer of negative electrode material, a negativeelectrode-side layer of electrical contact material.
 26. (canceled) 27.An electrochemical energy conversion cell according to claim 24, whereinthe one or more porous layers of the negative electrode-side structurecomprises, in addition to the functional layer of negative electrodematerial, a negative electrode-side layer of substrate material. 28.(canceled)
 29. An electrochemical energy conversion cell according toclaim 19, wherein the chemically unbound material in any one porouslayer is provided at a level in the range of about 0.1 to 65 vol % ofthe total solid content of the layer.
 30. (canceled)
 31. Anelectrochemical energy conversion cell according to claim 19, whereinthe chemically unbound material comprises free oxide selected from oneor more of SrO, CaO, BaO, MgO, Na₂O and K₂O.